Coating compositions of this type are generally known from, for example, U.S. Pat. No. 3,668,183, "Polyenamine Coatings: I. Formulation of Coatings and Determination Of Film Properties", K. L. Hoy and C. H. Carder, Journal of Paint Technology, Vol. 46, No. 591, April 1974, pages 70-75, and "Polyenamine Coatings: II. Chemical Methods of Improving Film Properties", C. H. Carder and H. O. Colomb, Jr., Journal of Paint Technology, Vol. 46, No. 591, April 1974, pages 76-78, referred to respectively as "Polyenamine Coatings I" and "Polyenamine Coatings II". Such coating compositions, when properly formulated and applied to a substrate, form a coating having highly desirable properties, such as hardness, flexibility and toughness. There is, however, a continuing difficulty in that such coatings frequently do not adequately adhere to the substrate on which they are applied.
U.S. Pat. No. 3,668,183 generally discloses the production of polyenamine resins by the reaction of polyacetoacetates with blocked polyamines. Also disclosed there is the possible use of a silicone component in the polyacetoacetate end cap to improve acid resistance.
WO-A-94/21738 discloses a crosslinkable liquid carrier-based coating composition comprising organic polymer(s) having acetoacetyl group functionality, a polyamine compound(s) having at least two acetoacetyl-reactive amino groups per molecule and a non-polymeric monoacetoacetyl compound(s) selected from a specific group of such compounds. The ratio of acetoacetyl groups of the acetoacetyl functional polymer(s) to the number of acetoacetyl-reactive amino groups of the polyamine compound(s) is between 0.5/1 and 2/1. The particularly preferred ratio is at or very near to 1.
In EP-A-199087, a coating composition is disclosed comprising a polyacetoacetate and a blocked polyamine crosslinker. The blocked polyamine and the polyacetoacetate are present in an amount such that the ratio of the number equivalents of primary and secondary amino groups of the polyamine to the number of equivalents of acetoacetate of the polacetoacetate is between 1/2 and 2. The polyacetoacetate has a number average molecular weight (Mn) of 1000-100,000.
In EP-A-417824 a polyacetoacetate with a molecular weight Mn between 200 and 1000 is disclosed. This polyacetoacetate is used as a crosslinker for amine-functional polymers. EP-A-417824 teaches use of an equimolar ratio of polymer and crosslinking agent.
The current invention provides a coating composition of the above-described type which, when cured, has a highly improved adhesion to the substrate. Other advantages include highly desirable cure-to-pot life ratio and corrosion-resistance properties. Further, it is possible to formulate the current compositions with low volatile organic content. Still further, the coating compositions herein disclosed are especially useful in the production of a multi-layer coating system, in particular for metal substrates, more particularly aluminum substrates, and more particularly, for aluminium for aircraft. In multi-layer coating systems, the coating compositions of the current invention are especially useful as the primer, and particular, primers which are free, or substantially free, of chromate pigment. As used herein, "primer" means any coating layer in direct contact with the substrate, after optional cleaning, conversion coating and/or other pre-treatment of the substrate.